The flexible pipes in most widespread use in oil mining are generally of the unbonded type in which the various successive and distinct layers have a certain freedom to move relative to one another and comprise, from the inside outward, a carcass consisting for example of an interlocked metal strip which serves to prevent the pipe from collapsing under external pressure, an internal sealing sheath made of polymer, a pressure vault consisting of at least one interlocked shaped wire wound in a spiral with a short pitch, armor layers known as tensile armor layers the lay angle of which, measured with respect to the longitudinal axis of the pipe, is less than 550°, and an external sealing sheath made of polymer. Such a flexible pipe is known as a rough bore pipe, the innermost element of which consists of a carcass that forms a rough bore, because of the discontinuities between the turns of the carcass.
When a flexible pipe comprises, from the inside outward, an internal polymer sealing sheath, a pressure vault consisting of shaped wires wound with a short pitch and intended to withstand the radial stresses caused by the flow of effluent along the flexible pipe, an anti-collapse sheath, one or more tensile and pressure armor layers wound around the anti-collapse sheath and a polymer external sealing sheath; it is known as a smooth bore because the innermost element is a smooth-walled sealed sheath.
In a variant, the flexible pipe has no pressure vault and the armor layers are spiral-wound in opposite directions with lay angles of around 55°. In this case, the internal and external pressures and the tensile forces also are exerted on or countered by these armor layers; such a flexible pipe is said to be balanced.
Smooth bore pipes are used to convey fluids that have no gaseous phase (and therefore mainly for water injection pipes); this is because, in the case of two-phase fluids, the diffusion, as it flows, of the gases through the internal polymer sealing sheath causes an increase in the pressure in the annulus around the internal sheath. When the pressure in the annulus becomes higher than the internal pressure in the pipe, for example during a decompression following a shutdown, the pressure difference may cause the internal polymer sealing sheath to collapse on itself. It is, amongst other things, in order to counter this risk that it is preferable in this case to have an anti-collapse carcass in the internal polymer sheath and therefore to use a rough bore pipe.
As recalled by the American Petroleum Institute standard API 17J, the carcass is a tubular layer, theoretically internal, formed of an interlocked metal winding and essentially intended to prevent the internal sealing sheath or the pipe from becoming crushed or collapsing under the effect of the external pressure when there is no internal pressure inside the pipe.
The carcass is most traditionally made of corrosion-resistant strip, generally shaped with a cross section similar to a couched “S” and wound in such a way as to produce turns that interlock with one another. In order to improve the performance of a carcass, various proposals have already been made. Reference may thus be made to document EP 0 429 357 by the Applicant company, which discloses a carcass in which the constituent strip comprises a wave-shaped part forming a bearing structure that increases the height of the cross section and therefore the second moment of area, thus improving the crushing strength of the carcass.
Carcasses made of shaped spiral-wound wire also exist in the prior art.
In application FR 01 13 748, the Applicant company has also proposed a hybrid carcass consisting of a winding with at least one elongate metal element such as a wire interlocked with a strip.
In all the cases considered and for which the invention is intended, the carcass therefore comprises, at least partially, a spiral-wound shaped wire or strip.
A problem has arisen with flexible pipes for producing and exporting gaseous hydrocarbons. This problem is associated with the flow of the gas in the rough bore pipe and more specifically with the phenomena of the formation of vortices which occur upon contact with the discontinuities between the turns of the carcass. What happens is that the surface discontinuity encountered at these discontinuities leads to the formation of vortices (and hence to vortex shedding) which disturbs the flow of the gas through the pipe. These vortices give rise to cyclic pressure variations which may lead to problems with resonance (vibration, noise) in the pipe and at the equipment and pipework on the platform (or the FPSO for “Floating production storage and offloading”) generally known as topsides, and also in submerged equipment. These pressure fluctuations and especially the resulting vibration may become very substantial and lead to problems of fatigue, particularly in said equipment and at the ends of the pipes which are then stressed more heavily than they were designed for. These problems may give rise to numerous undesirable phenomena such as, for example, leaks at the equipment and pipework connected to the pipe, as a result of fatigue.
One solution to these problems might be to alter the settings on the equipment of the platform or of the well, thus altering the conditions of use of the pipes in order to avoid these phenomena of resonance (reducing the pressure, the flow rate, etc.). However, while these changes to the conditions of use of the pipe allow the effects of these vortices to be reduced thereby reducing the problems of resonance, they do not allow the use of the pipe to be optimized nor do they truly solve the problem presented by these vortices.
Another problem that is noticed is that of the pressure drop in the flow of liquid or gas in rough bore pipes as a result of the turbulence created at the discontinuities.
The object of the invention is to find a solution to these problems of vortices and to propose a pipe of the rough bore structural type that allows gases to flow without exhibiting this detrimental turbulence or the associated resonance phenomena.